Rendering OSM data

Mountain-biking routes follow the same tagging schema as cycling routes
which is described in detail on the OSM wiki page about
cycling routes.
Relations that have the following tags are recognized as MTB routes:

A route tag with multiple values is supported when they are separated by
semicolon without any surrounding spaces.
The classification (and therefore the colour of the route in the map) is
determined from the network tag.

The label for each route is "guessed" from the tags in the following order:

If a ref tag exists, make a text label with the ref tag.

If a name tag exists, derive a reference from that, first by using
only upper-case letters and failing that by using the first letters of the name.